This invention relates to an apparatus for detecting an abnormality in means for sensing operating parameters of an internal combustion engine, and more particularly to an apparatus of this kind which is adapted to prevent erroneous diagnosis of detection of a fault in the operating parameter sensing means through monitoring of an output produced thereby when the rotational speed of the engine is below a predetermined value.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine is already known, which is adapted to determine the fuel injection period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of sensed values of engine speed and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients determined as functions of sensed values of engine speed, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
Among the engine operating parameters set forth above, the values of such parameters as engine speed, intake passage absolute pressure and throttle valve opening change from moment to moment in dependence on engine operation. These parameter values therefore have a major influence upon engine operating performance, particularly driveability during acceleration and deceleration, fuel consumption and exhaust emission characteristics, unless they are detected accurately promptly upon change. To this end, according to the above known method, the values of such engine operating parameters are detected upon generation of each pulse indicative of a predetermined rotational angle position, e.g., each pulse of a top-dead-center position (TDC) signal, and the fuel injection quantity is determined by the use of these detected values, to thereby assure required driveability of the engine.
In a fuel supply control system of the type described above, the output value of a parameter sensor for detecting the value of an engine operating parameter may show an abnormal value ascribable to a disconnection in the wiring or the like. If the fuel injection quantity is determined by using this abnormal value, it will adversely affect the engine operation to deteriorate the driveability, etc. since the quantity of fuel injected will no longer be of an appropriate value. In such cases, therefore, measures need to be taken to assure engine operation even when a failure develops in the parameter sensing system including the parameter sensor. There are also cases where the output of a parameter sensor indicates an abnormal value not only when a failure occurs but also temporarily owing to noise or the like. In such cases, suitable means are needed to distinguish between an actual failure in the parameter sensing system and an abnormality indication ascribable to transient causes such as noise. To cope with such phenomenon, a known data processing system disclosed in, e.g., Japanese Provisional Patent Publication (Kokai) No. 55-115101 is operable, when an output value from a parameter sensor is indicative of an abnormal magnitude, to measure by means of a timer the period of time the sensor continuously produces the abnormal value and, so long as the measured value does not prevail a predetermined period of time, to forgo diagnosing the output value as indicative of a fault and determine the fuel injection quantity by using a normal value which prevailed immediately before the abnormal value is indicated by the parameter sensor output. Also known from Japanese Provisional Patent Publication (Kokai) No. 54-141926 is a system for determining the fuel quantity by using a preset value in place of a parameter sensor output value when the sensor output value falls outside a predetermined range of normal values.
With the above-described apparatus, particularly the former, it can be avoided that a temporary abnormality in the output of a parameter sensor ascribable to noise or the like is mistaken as a failure in the parameter sensing system. However, in a case where the apparatus senses a parameter value in synchronism with a TDC signal, as described above, the following problems occur. Specifically, assume a situation in which the value of a parameter sensed at generation of a pulse of the TDC signal just prior to engine stoppage indicates an abnormality ascribable to inputting of noise or the like to the sensing system, and that the engine happens to be stopped before the next pulse of the TDC signal is generated. If such is the case, then the abnormal value resulting from the inputting of the noise or the like will be retained as long as the ignition switch of the engine is closed. Since the timer will continuously measure the duration of the abnormal value as long as the abnormal value continues, the measured value of time will eventually exceed the predetermined time period. As a result, the parameter sensing system will be diagnosed as faulty and an emergency operation such as a fail-safe action, which is intended to compensate for a failure when an abnormality actually occurs in the parameter sensing system, will implemented needlessly.